"The Free Cities and Solvunn were able to come up with it immediately themselves, once the Summoned were let loose. It may be closely guarded now, but it wasn't always, if they could find it."

Which means that the spell was somewhere out in the open enough that Thorne couldn't lock it down to their land alone. It was somehow able to be grasped and used by the leaders of two other factions entirely. Quickly. They knew what they were looking for. It doesn't mean the path to finding it will be easy for them now, but it was at one point known. A very specific summoning spell.

"The problem being here in Thorne is that they keep so much of their true information locked away from us." It is obvious that everything in the library and probably in the bookstores is approved-of texts. Nothing they don't want the Summoned or anyone else to be able to get.

"I have to wonder if it may be worth seeing if other factions might look into it as well. See if any of us can get lucky. I don't think they are sending anyone back, so it may be a one-way summoning, but it's worth knowing."